The invention relates to a method for producing seamless tubes with a three-roll bar rolling mill.
A generic method is described in the steel tube handbook (Publisher: Vulkan-Verlag, Essen, 12. Edition, 1995, p. 107-111).
Bar rolling mills which operate, for example, according to the continuous tube rolling process, are used in the production of seamless tubes. They are used to stretch a hollow block that was produced earlier by transverse rolling into a parent tube. This parent tube is subsequently reduced in a sizing or stretch-reducing mill to the desired final dimensions.
Basically, bar rolling mills exist in two embodiments, with two or three rolls per stand. The number of stands typically varies between four and eight.
It is known that a bar rolling mills are very sensitive to variations of the wall thickness and the diameter of the incoming hollow blocks. However, such variations cannot always be prevented in a transverse rolling process which is typically used to produce the hollow block.
In particular, transverse rolling mills with Diescher disks as a guide means produce hollow blocks with diameters that deviate in the head and foot region from the “filet region.” in the bar rolling process, these deviations can result in caliber underfills, wall thickness constrictions, holes and caliber overfills.
To minimize these errors, it is also known to arrange a hollow block reduction stand (void reduction stand) upstream of the bar rolling process. Such stand has four rolls in a two-roll bar rolling mill, and three rolls in a three-roll bar rolling mill.
Disadvantageously, in conventional hollow block reduction stands, the rolling conditions in the bar rolling mill still change with different diameters of the hollow blocks.
As a result, different input conditions are produced for the bar rolling mill during the deformation (input play hollow block to bar, reduction of the outside diameter in the first stand), which may again have negative effects for the quality of the tube.
It is an object of the present invention to define the a calibration and travel of the void reduction stand (VRS) for a three-roll bar rolling mill such that almost identical rolling conditions for the deformation in the bar rolling mill are retained even when the hollow block has different diameters.
It is hereby the goal to equalize as much as possible the diameter deviations in the hollow block as well as from one hollow block to another hollow block while simultaneously preventing underfilling or overfilling of the caliber.